Dear Peter,
The answer to your question is no. If A is related to B on the
paternal side and if A is related to C on the maternal side then B doesn't
necessarily need to be related to C. When in doubt, get more people tested
so that you have more data on which to base your conclusions.
Sincerely,
Tim Janzen

Regarding a situation where customers A, B, and C have Family Finder or
23andMe results ...
A matches B and C, but B and C don't match each other. Is it safe to say
that in reality that A is indeed a true match with B OR C (but not both B
and C)?